Isolation and characterization of naphthenic acids from a metal naphthenate deposit: Molecular properties at oil-water and air-water interfaces

Naphthenic acids from a West African metal naphthenate deposit have been isolated and characterized by infrared (IR), nuclear magnetic resonance (NMR), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The sample has been shown to comprise a narrow group of 4-protic naphthenic acids of molecular weight ∼1230 Da. The determined mass of 1230.0627 Da suggests a compound with the elemental composition C₈₀H₁₄₂O₈. The NMR data show no sign of carbon-carbon multiple bonds. Hence, the elemental composition indicates the presence of six saturated hydrocarbon rings. The naphthenic acids have proved to be highly oil-water (o/w) interfacially active. On elevation of the pH from 5.6 to 9.0, interfacial activity increases gradually due to a higher degree of dissociation of the carboxylic groups. At pH 9.0, the interfacial tension (IFT) between water and toluene-hexadecane (1-9 vol.) is lowered by ∼40 mN/m at concentrations of only.0050-0.010 mM naphthenic acid. The time rate of decrease of the IFT (dγ/dt) is also concentration-dependent, and a well-defined IFT is attained at long observation periods. The C₈₀ naphthenic acids form relatively unstable Langmuir monolayers. The stability decreases further with increasing pH as more monomers become dissociated and dissolve into the aqueous phase. The stability is altered upon addition of calcium ions into the subphase due to formation of calcium naphthenate at the surface. In the undissociated state, the acids have a molecular area of ∼160 Ų/ molecule in the noninteracting region. The high area reflects an extended molecular structure comprising four carboxylic head groups, which are likely to be separated by hydrocarbon chains.